[lammps-users] Negative pressure for water simulation

I am running MD simulation of water in spec model, i have done the minimization but i am getting nrgative pressure of about - 44053 bars after minimization. Is there any problem? How can i fix it to 1 bar?

Thank you.

In general you can use fix box/relax with a minimization to also adjust the box size and thus
aim for a target pressure. You can’t do minimization with fixed bonds or angles in an SPC water model.
I assume you are running with flexible bonds/angles.


What is the purpose of using flexible bonds/ angles. Is there any specfic command for that?

If you don’t use fix shake (or equivalent) then bonds and angles are springs and are flexible.
I was only asking b/c you mentioned spce water.

Again, you can’t currently do a minimization in LAMMPS with rigid bonds or rigid bodies.
It will not preserve bond lengths.


I am running MD simulation of water using spc/e model.
First i have done the minimization, after minimization presssure is negative( huge) then i gave temp of 300 by velocity command then i used fix npt as:
fix 1 all npt temp 300 300 100 iso 1 1 100
Temp is stabilized but pressure is still not comming to 1 bar and also there is huge fluctuation in the pressure till last step its far away from 1 bar… Please suggest a solution.

I have used the box /relax command and the pressure after minimization is positive now.
I have used as
fix 1 all box/relax iso 0.0 vmax 0.001
But the pressure after minimization is 714 bars. Is that any problem with my etol or number of interations or box/relax command.?